Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz et al., Acc. Chem. Res., 2001, 34, 5, p.359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation, i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
Compounds known in prior art which have been shown to be effective p-type semiconductors for organic FETs are dithieno-hiophene (DTT) (1a) and its fused dimer α,α′-bis(dithieno 3,2-b:2′,3′-d]thiophene (BDT) (1b) having the structures shown below [see H. Sirringhaus et al., Appl. Phys. Lett., 1997, 71, 26, p.3871; Li et al., J. Am. Chem. Soc., 1998, 120, p.2206; J. J. Morrison et al., Synth. Met., 1999, 102, p.987].

In particular BDT, which has been extensively studied, has been shown to be an effective p-type semiconductor for organic FETs with a very high charge carrier mobility between 1×10−3 and 5×10−2 cm2V−1s−1 and very high current on/off ratios (up to 108). BDT also has been found in the solid state to have a completely coplanar formation, and to be more planar than oligomers of thiophene.
However, BDT has a high melting point and is very insoluble, therefore, if used as the active layer in an organic thin film transistor, it cannot be readily solution processed. As a result, for applications like FETs, prior art materials like BDT are usually deposited as a thin film by vacuum deposition, which is an expensive processing technique that is unsuitable for the fabrication of large-area films. To improve the solubility of BDT, several substituted derivatives have so far been synthesized (1c), [see J. J. Morrison et al., Synth. Met., 1999, 102, p.987] but these have still required vacuum processing when used in thin film transistors.
It is an aim of the present invention to provide new materials for use as semiconductors or charge transport materials, which are easy to synthesize, have high charge mobility and are easily processible to form thin and large-area films for use in semiconductor devices. Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors have found that these aims can be achieved by providing new aryl copolymers of 9-H,H-fluorene.
9,9-Dialkylfluorene (2) containing (co)polymers have been extensively investigated as a class of soluble material for organic light emitting diode (OLED) applications [see M. T. Bernius et al., Adv. Mater., 2000, 12(23), p.1737; U. Scherf et al., Adv. Mater., 2002, 14(7), p.477]. For OLED applications, it is desirable to prevent aggregation of the polymer backbones which can lead to excimer quenching. The 9,9-dialkyl substituents help to provide solubility for the polymer and also pack orthogonal to the polymer backbone, thus helping to prevent aggregation of the polymer chains.

U.S. Pat. No. 5,708,130 and U.S. Pat. No. 6,169,163 disclose polymers of 9,9′-disubstituted fluorene as shown in structure (3), wherein R is C1-20-hydrocarbyl that may also contain hetero atoms. WO 00/22026 discloses polymers of 9,9′-disubstituted fluorene wherein R is aryl or heteroaryl. U.S. Pat. No. 6,353,083 discloses copolymers of 9-substituted or 9,9′-disubstituted fluorene with two other different monomers. These documents further suggest to use the polymers for OLEDs.

Bin Liu et al., Macromolecules, 2000, 33, p.8945 report the synthesis of copolymers of 9,9′-dihexylfluorene and decylthiophene and the investigation of these materials for OLED applications.
WO 02/45184 discloses a field effect transistor (FET) comprising an organic semiconductor and a binder, wherein the semiconductor may, inter alia, comprise polymers of structure (3) above, wherein R is selected from H, alkyl, aryl or substituted aryl.
H. Sirringhaus et al., Appl. Phys. Lett., 2000, 77(3), p.406 report poly-9,9-dioctyl-fluorene-co-bithiophene (4, F8T2) to show reasonable mobility (10−2 cm2V−1s−1) in a field effect transistor despite the presence of octyl chains on the Sp3-carbon of the fluorene bridge. This good mobility is a consequence of the liquid crystalline behaviour of the polymer, which allows control of the morphology of the semiconductor in the transistor. Specifically, the polymer is annealed in its nematic liquid crystal phase at 275–285° C. and aligned on a rubbed polyimide layer in the direction of charge transport before quenching to ‘freeze in’ the order.

However, fluorene co-polymers of prior art which are substituted at the sp3 carbon atom in 9-position with hydrocarbon groups have the disadvantage that the sp3 dialkyl bridge disrupts close packing, which negatively affects charge transport.
In contrast, the present invention relates to unsubstituted fluorene co-polymers which are not substituted at the sp3 carbon in 9-position of the fluorene bridge but retain mesogenic behaviour. The absence of chains at the fluorene bridge facilitates closer packing of the polymer backbones and improved charge transport. Solubilising side-chains can now be present on, for example, a thiophene co-monomer, and are thus able to lie in the plane of the polymer backbone, therefore not disrupting the packing of polymer chains. Furthermore, this approach affords polymers which exhibit liquid crystalline behaviour at lower temperatures. Thus, some of the materials of the present invention exhibit liquid crystallinity already at temperatures around 150° C., which is considerably lower than, for example, the polymer F8T2 of prior art, which has a nematic phase at a temperature of 265° C. or higher, and enables lower processing temperatures, e.g. in the fabrication of FET devices.
A further aspect of the invention relates to reactive mesogens having a central core comprising 9-H,H-fluorene and arylene units, said core being linked, optionally via a spacer group, to one or two polymerisable groups. The reactive mesogens can induce or enhance liquid crystal phases or are liquid crystalline themselves. They can be oriented in their mesophase and the polymerisable group(s) can be polymerised or crosslinked in situ to form polymer films with a high degree of order, thus yielding improved semiconductor materials with high stability and high charge carrier mobility.
A further aspect of the invention relates to liquid crystal polymers like liquid crystal main chain or side chain polymers, in particular liquid crystal side chain polymers, obtained from the reactive mesogens according to the present invention, which are then further processed, e.g., from solution as thin layers for use in semiconductor devices.